The present invention relates to control systems for a printing press.
The mainstream of color printing encompasses the use of four printing inks (cyan, magenta, yellow and black). These inks are printed on paper one at a time to form a four color image. The printer will also print a series of single ink control targets in the page margin to aid in the control of the ink feed rate. Densitometers are commonly used to measure the cyan, magenta, yellow and visual densities from these areas. Conversions may exist to convert these corresponding density values to estimates of the ink present. However, when measurements are taken from a single ink control target, image quality cannot be inferred since differences in dot gain will not contribute to the solid ink densities measured by such a device. Other image quality variances caused by press behaviors such as ghosting, will not be detected in the densities measured from the control target. In addition, it is undesirable to use a control target due to the extra paper required for its printing and the effort needed to remove it from the final product.
Color monitor systems exist that measure the image area, where multiple inks are present. These systems use three channel sensors such as a color camera. These systems generally provide an overall score indicating the quality of color match between reference and production copies. The quality of match is generally presented as the difference in sensor space of red, green, blue or cyan, magenta and yellow between these two copies. When an operator is presented with this information, human judgment is involved in translating this information to the control of a specific ink on the printing press. Automated translations from the color difference to the ink content do not exist. This occurs because the working range of the device is limited to the visible region of the spectrum where the black ink and three color process black are indistinguishable. Since the relationship between the color separation and the quantity of ink being printed is not known, systems of this type provide little aid to the operator for control purposes. Monitor systems such as this do not provide closed loop color control.
The technology used in prepress color separation cannot be used in color control of a printing press. In a prepress color separation, an original image is scanned with a three channel color sensor providing reflection values in the red, green and blue spectral regions. The color separation process also determines the amount of cyan, magenta and yellow ink that would be needed to reproduce this original image on a printing press. Knowing that the combinations of these three inks ideally produce black, an algorithm in the color separation system will substitute black for the three inks of cyan, magenta and yellow. This process is usually called Gray Component Removal (GCR). The magnitude of each of the three inks that are subtracted as black is added varies depending on the impurities in the process inks. There are therefore many different combinations of cyan, magenta, yellow and black that can be used to reproduce the original. The task of this prepress color separation is to find one of the color combinations that will reproduce the original image. This can be accomplished by operator preference or other factors. The output of the process does not imply the amount of ink (if any) used on the original scanned image, nor does it imply the amount of ink that has been applied to the paper in an actual printing condition.
Since three channel sensors have posed a problem in determining the quantity of four inks on a printed page, some have devised a four channel sensor in anticipation of overcoming the problem. U.S. Pat. No. 3,376,426 to Frommer (1968) discloses a density monitor system which includes an infrared sensor for detecting the presence of black ink and a linear 4-by-4 subtractive suppression matrix for reducing a secondary affect caused by the impurities of the inks. It is suggested by Frommer that the linear matrix is adjusted for a correct response at one maximum level of density. Since neither the dot gain, the trapping nor the relation between the ink volume and the light reflectance is in a linear form, the output of Frommer's suppression matrix does not represent the amount of ink present on the paper, nor is it his intention. When one or more color inks are mixed with the black ink, the output of this suppression matrix for color inks will exhibit a large degree of error. This large error can cause an ink control system to make adjustments to inks which are not requiring adjustment, or to move inks in the wrong direction, resulting in an unstable system. For this reason, Frommer's system cannot be used to control a printing process in a stable and accurate manner, nor preset the printing press based on a given reference.
U.S. Pat. No. 4,649,502 to Keller (1987) discloses a demask process to determine the surface coverage for each ink by solving extended Neugebauer equations iteratively. The extended Neugebauer equations used in Keller's processing are pure stochastic models which give no consideration to dot gain (called "point increment" by Keller) and trapping factors for different dot sizes. Thus, Neugebauer equations become less accurate, especially when the dot size of one or more inks is in the vicinity of 50 percent, or one ink has a low dot size while the other three inks have high dot sizes. Therefore, Keller's process must further include weighting matrices G1 and G3 to compensate for errors caused by the dot gain and the over-printed colors, respectively. However, these two compensation functions are only used for assigning lower weighting factors to data with lower confidence levels, but not for eliminating the existing errors. Thus, the error produced by the demask process will propagate into the ink feed rate calculation step making the color adjustment processing less accurate. Furthermore, Neugebauer equations describe light reflectance as a function of the ink coverage values, which are independent variables in Neugebauer equations. There is no easy way to reverse the Neugebauer equations so those ink coverage values, which we want to know, can be described explicitly as dependent variables. To find these surface coverage values, a long and complicated iterative process has to be used to solve these multi-variable non-linear simultaneous equations. This process is very time consuming and has to be performed for a large number of measurement elements. This process is also risky since the coefficient matrices of these equations may be singular or ill-conditioned for certain reflectance combinations resulting a very poor solution, or none at all.
In order to adjust the color of currently printing copies on the press so they look substantially like a reference, information should be known about the ink content of the reference and that of the currently printed copy. Adjustments could then be made in the ink feed rate based on this difference. An ink separation process is able to take the sensor data presented when viewing an area with multiple inks and convert it into actual ink content on the page.